Molecular and Metabolic Analyses in Developing Olive Fruit in Relation to Different Water Regimes

Despite the global economic importance of olive (Olea europaea L.), little is known about the molecular and metabolic changes during fruit development and the modulation of quality-related metabolic pathways during drought stress. In this work, we report the expression pattern of genes involved in important pathways of secondary metabolism (polyphenols, terpenoids) during fruit development in rainfed or fully irrigated olive plants. Phenolic compounds represent a complex mixture in both olive fruits and oil. The interest on these compounds is due to their demonstrated anti-atherogenic and anti-cancerogenic effects providing to the olive oil important nutraceutical properties. Several parameters affect olive fruit phenolic content and these include genotype, pedo-climatic conditions, agronomic techniques (e.g. irrigation) and the develop¬mental stage of the drupe. To gain insight into the transcriptional regulation of phenolic pathways we analysed the pattern of expression of important genes during olive fruit development. Transcript levels of phenylalanine ammonia lyase (PAL), chalcone synthase (CHS), and dihydroflavonol reductase (DFR) genes showed different expression patterns during fruit development. CHS and DFR showed stronger expression in the skin than in the pulp. PAL expression was strong before pit hardening and decreased thereafter. CHS expression peaked at pit hardening and at veraison stage, whereas DFR was only strongly expressed at fruit ripening, suggesting a crucial role for these genes in the regulation of flavonoid biosynthesis in the last developmental stages of olive fruit. Water stress enhanced the expressions of PAL and CHS at pit hardening and of DFR in the skin at ripening. Of the terpenoids, amyrin synthase (AS) showed a decreasing expression trend throughout development, whereas lupeol synthase (LUS) was highly expressed in the skin of ripe fruit. Induction of LUS gene expression was observed at the early developmental stages in fruit of water-stressed plants. To gain insight into the metabolic changes in the fruit under different water regimes in the field, we initiated, in a high-throughput approach, analysis of the mesocarp of ripe fruits by GC-TOF mass spectrometry, and differences in metabolite composition were determined.